Coated steel cladding provides a cost-effective
weathering envelope for buildings.
Peter Mayer of Building LifePlans examines
the specification options and whole life costs.
Coated steel cladding comes as profiled steel sheets or composite panels, which are used to clad walls and roofs. It is used for cladding a large proportion of industrial, warehouse and out-of-town retail units.
The challenge for coated steel cladding manufacturers is to make a product that both resists corrosion and looks good. To achieve this, coated steel cladding is a composite laminated product comprising:
• Steel sheet 0.6 – 2mm thick, which on its own would rapidly rust in the UK climate.
• A corrosion protection layer; zinc or zinc–aluminium alloy applied by a hot–dip process.
• A paint finish: an organic coating to provide an attractive finish usually applied by a roller, typically based on Polyvinyl–chloride (PVC or Plastisol), Polyvinylidene–fluoride (PVDF or PVF2), Polyester or Polyurethane formulations.
The challenge for specifiers is how to determine which coating system offers the best performance.
Steel sheet and corrosion protection
Steel sheet should be to BS EN 10327 the European standards for continuously hot-dip coated steel sheet. This standard confirms steel quality, minimum thickness, surface quality and adhesion criteria for zinc and zinc–aluminium alloy coatings.
A roll of steel is cut to form cladding sheets, at these cut edges bare steel is exposed. The exposed steel face is prevented from corroding by zinc layers which corrode sacrificially in preference to the steel. The sacrificial zinc layer initially protects the cut edges but eventually these will corrode, leading to delamination of the organic coating layer.
Typical corrosion–protection coating thicknesses are 10 microns on each side of the steel. This is obtained by coating masses of 275g/mÏ (Z275) for zinc and 255 g/mÏ (ZA 255) for aluminium–zinc alloy. Aluminium–zinc alloy gives better edge performance with a reduced corrosion rate and better paint adhesion.
The thicker the corrosion protection layer the less likely and the longer the time before for the onset of delamination. But it is not as simple as this, as production processes and preparation of the steel play an important role in the long-term performance of steel cladding.
The properties required for a durable organic coating are good adhesion, resistance to ultraviolet light ,resistance to chalking and abrasion resistance. Part three of the European standard EN 10169 will eventually provide a useful method to compare components made from organically coated steel sheet. Unfortunately the standard is still in draft form.
In the meantime, specifiers have to rely on test data from manufacturers or third-party certification. Alternatively a detailed assessment of manufacturer’s guarantees may provide assurance of performance.
Organic coatings have a finite life depending on how these are maintained. Failure is generally an issue of appearance as the weatherproofing function of the cladding is rarely compromised because of the underlying corrosion-resistant layer. However this is thin and corrosion will eventually set in unless the cladding is regularly repainted.
Polyvinyl–chloride (PVC or Plastisol)
PVC based coatings are applied in thicknesses up to 200microns. Evidence shows these can last up to 40 years.
Polyvinylidene–fluoride (PVDF or PVF2)
PVDF coatings are thinner at 25microns with expected performance in excess of 20 years.
A thin organic coating 25 microns with expected performance in excess of 15 to 20 years.
An organic coating of 50 microns thickness with expected performance in excess of 20 years.
Some durability issues apply to all types of organic coated cladding. The amount of time the coating lasts before painting is needed is largely related to the ultraviolet light dosage:
• Colour: Lighter colours have longer periods between redecoration than darker colours.
• Building element: Cladding on flat roofs tends to deteriorate faster than claddings on pitched roofs. Cladding on walls gives the longest intervals between repainting.
• Orientation: South facing claddings tend to deteriorate slightly faster than east, west or north facing claddings.
• Environment: Polluted or marine environments will tend to accelerate paint deterioration.
• Cladding profile shape: Flexible organic coatings are less likely to deteriorate and crack at shaped edges of profiled sheeting.
(Based on mild steel sheet minimum thickness 0.7mm; hot–dip galvanized to BS EN 10327, 275g/mÏ zinc coating weight)
Polyester based coating, minimum 25 microns nominal thickness.
Polyurethane based coating, minimum 50 microns nominal thickness.
• Costs include cleaning, maintenance, recoating and repairs. Cost and frequency of maintenance are generic and are based on 1000mÏ of simple cladding. Repainting is modelled at an average of the range given. The model assumes the steel cladding is not replaced during the 60 year period.
• A discount rate of 3.5% is used to calculate net present values.
• A cost analysis based on project specific information is essential for a realistic best value appraisal.